What is the average global surface temperature ?
A. ${12}^\circ$
B. ${17}^\circ$
C. ${11}^\circ$
D. ${15}^\circ$

Hint: The term greenhouse effect has been derived from a phenomenon that occurs in a greenhouse. The greenhouse effect is a naturally occurring phenomena that is responsible for heating of the earth surface and atmosphere.

Complete Answer:
- A man made greenhouse looks like a small glass house and is used for growing plants especially during winter. In a greenhouse the glass panel lets the light in but does not allow heat to escape. Therefore the greenhouse homes are very much like inside a car that has been parked in the sun for a few hours. Our earth also behaves as a greenhouse due to some greenhouse gases and it is responsible for the temperature of the earth.
- If the greenhouse effect was not present then the temperature of earth would have been minus 18 degree Celsius. But for now the average global temperature is 15 degrees Celsius.
- In order to understand the greenhouse effect it is necessary to know the fate of energy of sunlight that reaches the outermost atmosphere. Clouds and gases reflect about one-fourth of the incoming solar radiation and absorb some of it but almost half of the incoming solar radiation falls on earth surface heating it while a small portion is reflected back.
- Earth surfaces re emit heat in the form of infrared radiation but part of this does not escape into as atmosphere gases example carbon dioxide and Methane absorb a major fraction of it. The molecules of these gases radiate heat energy, a major part of which again comes to earth surface heating it once again. And this following process maintains the temperature of the global surface of Earth.

Hence the correct answer is option D.

Note: Increase in the level of greenhouse gases head lead to considerable heating of earth leading to global warming.
During the past century the temperature of earth has been increased by 0.6 degree Celsius.